things arose by separation,
the parts once formed remaining unchanged. The earth was cylindrical
in shape, suspended in the air in the centre of the universe, and the
stars and planets revolved around it, each fastened in a crystalline
ring; the moon and sun revolved in the same manner, only at a farther
distance. The generation of the universe was by the action of
contraries, by heat and cold, the moist and the dry. From the moisture
all things were originally generated by heat. Animals and men came
from fishes by a process of evolution. There is evidence in his
philosophy of a belief in the development of the universe by the action
of heat and cold on matter. It is also evident that the principles of
biology and the theory of evolution are hinted at by this philosopher.
Also, he was the first to observe the obliquity of the ecliptic; he
taught that the moon received its light from the sun and that the earth
is round.

Anaximenes, born at Miletus 588 B.C., asserted that air was the first
principle of the universe; indeed, he held that on it "the very earth
floats like a broad leaf." He held that air was infinite in extent;
that it touched all things, and was the source of life of all. The
human soul was nothing but air, since life consists in inhaling and
exhaling, and when this is no longer {218} continued death ensues.
Warmth and cold arose from rarefaction and condensation, and probably
the origin of the sun and planets was caused by the rarefaction of air;
but when air underwent great condensation, snow, water, and hail
appeared, and, indeed, with sufficient condensation, the earth itself
was formed. It was only a step further to suppose that the infinite
air was the source of life, the god of the universe.

Somewhat later Diogenes of Apollonia asserted that all things
originated from one essence, and that air was the soul of the world,
eternal and endowed with consciousness. This was an attempt to explain
the development of the universe by a conscious power. It led to the
suggestion of psychology, as the mind of man was conscious air. "But
that which has knowledge is what men call air; it is it that regulates
all and governs all, and hence it is the use of air to pervade all, and
to dispose all, and to be in all, for there is nothing that has not
part in it."

Other philosophers of this school reasoned or speculated upon the
probable first causes in the creation. In a similar manner Heraclitus
as